Conventionally known in the art is a keyboard assembly for playing music automatically by physically actuating the keys by means of key actuator devices to play back musical notes automatically along with the music playing data signal. An example of such a keyboard assembly is disclosed in registered Japanese utility model publication No. 2,555,777.
The keyboard assembly disclosed in the above referenced publication comprises a plurality of keys, each supported swingably on a key fulcrum, and a same plurality of actuator devices of a solenoid plunger type, each disposed in correspondence to each of the keys. The solenoid plungers are arrayed alternately one after another (zigzag) in two rows in the direction of the key juxtaposition. The tip of the plunger member included in the solenoid plunger pushes up the rear end part of the corresponding key from underneath to actuate the key to its depressed position.
More specifically, the actuator devices for the odd-numbered keys (C, D, E, F#, G#, A#) in an octave are arrayed in one of the two rows (e.g. the rear row), while the actuator devices for the even-numbered keys (C#, D#, F, G, A, B) in an octave are arrayed in the other of the two rows (e.g. the front row), in order to realize an efficient disposition of the actuator devices. Further, each actuator device is disposed under each corresponding key at its laterally central position, i.e. on the center line of the width (the dimension in the direction of the key juxtaposition) and accordingly all the actuators are arrayed with the same spacings as all the keys.
It should be noted, however, that the spacings among the keys including twelve keys (seven white keys and five black keys) per octave are not the same in the octave with the keyboards for ordinary keyboard musical instruments, except the keyboards for some types of toy musical instruments. The key spacing between the adjacent keys among the C through E keys is wide and that among the F through B keys is narrow in the ordinary keyboard. Thus, the actuator devices are to be disposed with the wide spacing for the C through E keys and with the narrow spacing for the F through B keys.
The actuator devices are usually designed with the common dimensions for all the keys so as not to increase the number of model kinds, and in addition the dimensions are determined so that the actuator devices can be located in a limited area in the keyboard assembly. This will limit the size of the actuator device to be accommodated in the region of the narrow key spacing.
The bigger the actuator device is sized, the higher efficiency per power consumption will be obtained. In other words, the inductance L of a solenoid coil is proportional to the square of the cross-sectional area of the core (or the ring of the wound coil) and the square of the number of turns of the coil, and is inversely proportional to the length of the magnetic path. This means, a bigger-sized actuator device can contain a core having a larger cross-sectional area, which in turn shows a larger inductance L, thereby giving higher efficiency. In order to make the electric power to be a desired value at the initial moment of the current flow through the coil, the larger the inductance L is, the smaller the required electric current is.
As in the case of the conventional keyboard assembly, where there are size limitations for the actuator device, a small-sized actuator device would necessitate a bigger power supply source to secure a necessary actuation force. A bigger power supply, however, will be disadvantageous from the viewpoint of electric power consumption as well as from the viewpoint of heat generation by the coil. This will discourage the miniaturization of the entire keyboard assembly.
Moreover, in the conventional keyboard assembly, the disposition of the actuator devices is completely dependent on the disposition of the keys, which provides little freedom for designing. There may be the necessity of providing fixing members such, as screws to fix the actuator devices or the yokes to the keyboard assembly and providing various components such as a temperature sensor for a fail-safe system. But, as the actuator devices are to be disposed under such restrictions, it is difficult to secure spaces as well as to find optimum places for providing those various components.
Also known in the art is a keyboard assembly comprising swing weights, each for each key, for introducing inertia in the key movement by swinging as interlocked with the key, in which the actuator device actuates the corresponding swing weight, which in turn actuates the corresponding key to swing to the depressed position. In such a keyboard assembly, where the keyboard frame is made of plastic material, several ribs will be provided to reinforce the structure, which will require the swing weights to be positioned by circumventing the ribs. Thus, the swing weights cannot be arrayed with equal spacing, which causes a wider space region and a narrower space region. As a result in this case, the disposition of the actuator devices with respect to the swing weights contains the same problem as mentioned above in connection with the disposition of the actuator devices with respect to the keys.